The present invention relates generally to drop-on-demand liquid emission devices such as, for example, ink jet printers, and more particularly such devices which employ an electrostatic actuator for driving liquid from the device.
Drop-on-demand (DOD) liquid emission devices with electrostatic actuators are known for ink printing systems. U.S. Pat. Nos. 4,520,375; 5,644,341 and 5,668,579 disclose such devices having electrostatic actuators composed of a diaphragm and opposed electrode. The diaphragm is distorted by application of a first voltage to the electrode. Relaxation of the diaphragm expels an ink droplet from a nozzle orifice. Other devices that operate on the principle of electrostatic attraction are disclosed in U.S. Pat. Nos. 5,739,831, 6,127,198, and 6,318,841; and in U.S. Pub. No. 2001/0023523. According to the prior art, an electrostatic attraction force is applied in a single direction, as the electrodes can only attract; repulsion being impractical. Thus, the devices must rely on the elastic memory of the diaphragm to return to an at-rest position.
Devices that rely on the elastic memory of the diaphragm to expel liquid drops exhibit a reduction on the liquid-expulsion force over the time period that liquid is being expelled. That is, the speed at which the diaphragm moves as it approached its at-rest position decreases. The result is a tendency for liquid to be expelled at a greater velocity at the beginning of the time period and a lesser velocity at the end of the time period. This often results in the production of undesirable satellite droplets following a main drop.
It is known that the force that expels drops from the emission device also causes some liquid to flow backward toward the liquid reservoir. The backward flow of liquid diverted from the nozzle orifice further reduces the velocity of the liquid being emitted from the nozzle orifice.
A drop-on-demand liquid emission device, such as for example an ink jet printer, includes a member movable through a path for driving liquid from the device, wherein the speed at which the member moves is reduced over the time period that liquid is being expelled. During that time period, a portion of the liquid flows through a passage away from the nozzle orifice. According to a feature of the present invention, a variable flow restrictor increases the resistance to flow through the passage during the time period that liquid is being expelled; thereby tending to compensate for the reduction of the liquid-expulsion force over the time period. The result is a reduction of undesirable satellite droplets following a main drop.